Up to 25% of shareholders are expected to protest against Rupert, James and Lachlan Murdoch retaining their positions on the News Corporation board at the embattled media group's annual meeting in Los Angeles next week.

A string of advisory bodies and corporate governance campaigners have issued recommendations to vote against up to 13 directors on the 15-strong board, the composition of which has been the subject of shareholder concern for some time.

The phone-hacking allegations have further focused investors on the structure of the board – where there are concerns about the limited power and influence of independent directors.

Rupert Murdoch is both chairman and chief executive, which is reasonably common in the US but often unpopular with UK-based investors.

The Murdochs control around 39% of the votes at the company, which means the directors would not be voted off even though they own just 12% of the company, because of the complex share structure.

Hermes Equity Ownership Services, which votes on behalf of the BT pension fund and other investors, became the latest shareholder advisory service to advise its clients not to support all the directors on the NewsCorp board.

Explaining its decision to withhold support from five directors – Rupert, James and Lachlan Murdoch, Arthur Siskind and Andrew Knight – Jennifer Walmsley, director of Hermes EOS, said: "News Corp has not reacted with sufficient urgency to investor concerns about its board composition and corporate culture.

"The time is right for the company to appoint an independent chairman to rebuild trust, help correct the governance discount, and ensure that the interests of all investors are properly represented," she said.

The firm, along with others, is considering sending a representative to the meeting in Los Angeles where a small shareholder, Christian Brothers Investment Services, intends to table a resolution from the floor calling for Murdoch to split his role of chairman and chief executive.

Industry sources believe that opposition to the directors could reach 25%, given the array of other investor bodies and advisory agencies that have also made recommendations to oppose directors."It is possible we could see a significant protest vote of 25% or so," one source said.

Other comments have been made by the following bodies:

• The Local Authority Pension Fund Forum (LAPFF) wants Rupert and James Murdoch to be replaced by former British Airways boss Sir Rod Eddington.

• Pirc, the UK advisory service, is advising a vote against 11 directors.

• The Australian Council of Superannuation Investors (ACSI) wants Rupert Murdoch to be replaced as chairman by someone "genuinely independent".

• Calsters – the California State Teachers' Retirement System, one of the largest public sector pension funds in the US – has already voted against 13 directors on the News Corp board.

Well-known US advisory firms are also recommending voting against the Murdochs. Glass Lewis, which advises institutions holding $15tn (£9.6tn) worth of investments, has recommended that investors vote against the re-election of James and Lachlan Murdoch, while Institutional Shareholder Services, a major advisory agency for shareholders, has recommended voting against 13 of directors on the News Corp board.

News Corp said it "vehemently disagrees" with advice from the advisory firms about failing to back its directors and pointed to its strong fiscal performance for 2011, with revenues up 2% to $33.4bn and operating profit up 23% to $4.85bn.

The company also stressed that it had a "strong board of directors".

"Our slate of director nominees consists of sophisticated, world-class directors who serve to enhance our board of directors … We disagree with recommendations that stockholders should vote against our directors. We respectfully request that stockholders vote for all directors as the board has recommended," News Corp said.